ANALYSIS OF THE IMPACT OF CITRIC ACID CIP CARRYOVER ON DISINFECTION BY-PRODUCT FORMATION IN MEMBRANE TREATMENT FACILITIES.

Transcription

1 ANALYSIS OF THE IMPACT OF CITRIC ACID CIP CARRYOVER ON DISINFECTION BY-PRODUCT FORMATION IN MEMBRANE TREATMENT FACILITIES Thomas F. Munding, OPUS International Consultants Ltd., Harbourside Drive, North Vancouver, BC V7P 3S1 Ph: Walter J.G. Bayless, OPUS International Consultants Ltd., North Vancouver, BC Abstract Contamination of membrane systems treated water from clean-in-place (CIP) chemicals is unavoidable. Citric acid is of particular concern as it will react with chlorine disinfection in the downstream process resulting in formation of DBP. In order to estimate the quantity of citric acid carryover at a WTP, a series of bench-scale tests were conducted indicating a citric concentration of 1.8 mg/l to the plant clearwell with corresponding increase in THM (chloroform) from 39 µg/l to 50 µg/l and an HAA increase from 35 µg/l to 63 µg/l. Reduced citric acid concentration in the treated water resulted in even higher THM formation to a maximum of 56 µg/l since the ph was not as low. For source waters with higher alkalinity, buffer capacity can mask the presence of citric acid carryover. A review of the existing CIP cleaning process was undertaken to identify options to reduce carryover and an improved method of flushing the CIP solutions from the membrane is proposed whereby the CIP solution is flushed by air-purge displacement, rather than traditional dilution flushing. The overall impact on the water supply due to CIP carryover is likely nominal on an annualized basis. However, short term spikes in THMs and HAAs may result in sample failure in the distribution system depending the ph and citric acid carryover. Changes to the CIP flushing procedures could reduce these spikes, risk of sample failure and potentially reduce the volume of flush water required. Introduction Membrane UF/MF drinking water treatment plants typically use citric acid as one of their CIP membrane cleaning solutions. After membrane cleaning, the CIP solution is flushed and drained from the membranes, process piping and vessels before the system is returned to potable water production. However, the treated water produced immediately after CIP will be unavoidably contaminated by trace amounts of any CIP solution which has not been fully purged from the membrane system. Normally such CIP solution carryover to the treated water is assumed to be of such insignificant quantity as to have negligible effect on the quality of the treated water. Eagle Lake WTP is a submerged UF membrane facility located in British Columbia designed primarily for removal of organics and color. Typical of many water sources in the Pacific Northwest, Eagle Lake has very low alkalinity and thus has little buffering capacity for ph. During operation of this plant the operators observed a temporary drop in the treated water clearwell effluent ph after each membrane unit CIP with citric acid. This is an indication that after draining and flushing of the CIP solution, citric acid is remaining in the membrane system and is being carried-over to the treated water when the membrane unit is returned to service. Flushing water to the membranes was increased to minimize any CIP solution carryover, 1

2 however, the ultimate quantity of flushing water that can be used is limited by the capacity of the receiving CIP and neutralization tank volume. At maximum achievable flushing volume the resulting clearwell ph depression is approximately 0.2 to 0.3 ph units. The clearwell effluent ph would then recover to its normal value after several hours of treated water production as the citric acid carryover is purged through the UF unit and discharged from the clearwell. Citric acid carryover to the treated water is a concern because it will react with chlorine disinfection to form DBPs like THM (primarily chloroform) as well as HAAs. A basic sampling and test procedure was conducted to determine the scale of DBP formation from citric acid carryover at this plant. Method A large sample volume (20 L) was collected from the Eagle Lake WTP clearwell effluent several days after any UF unit had a CIP cleaning to ensure it was well flushed of any possible residual citric acid. This sample was collected on Monday, June 6, 2016 at 9:20 am. The clearwell effluent was chosen as an appropriate sample point as this provided a good representation of the clearwell contents during normal operation, including; the chlorine residual remaining after the initial chlorine demand, orthophosphate dosing added for corrosion control, and caustic dosing added for ph correction. This sample was then used to run several citric acid titration tests to determine the following information: 1) What is the concentration of citric acid going to the clearwell (ie. how much carryover)? 2) What is the expected magnitude of DBP formation that could be attributed to citric acid carryover? A 1 L sample of unchlorinated water was taken from a running UF unit s permeate pump discharge sample port for use to make up dilute citric acid solution for use in the citric acid titrations. Liquid citric acid (50% concentration, SG = 1.23) was taken from the plant s CIP dosing system of which 0.6 ml was added to the 1 L sample while being mixed with a magnetic stirrer. This dilute citric acid solution was measured to have ph = 3.03 (@ 9:40 am). Titration tests showed that approximately 5 ml of the dilute citric acid solution was required in order to reduce the ph value of 1 L treated water sample by 0.2 to 0.3 ph units. Figure 1 below shows the resulting water sample ph vs Citric Acid addition with straight line interpolation between the data points. The 24 hour ph value for test #2 was ignored and assumed to be result of some measurement error. From the 20 L sample jug, seven 1 L samples were taken for citric acid DBP formation tests as follows: 1) Reference 1 L sample (no citric acid), ph and free chlorine were measured and sample bottle sealed at 9:25 am 2) 1 L sample with 5 ml of the dilute citric acid slowly added with magnetic stirring, ph measured and sample bottle sealed at 9:55 am 3) 1 L sample with 12 ml of the dilute citric acid slowly added with magnetic stirring, ph measured and sample bottle sealed at 10:05 am 4) 1 L sample with 16 ml of the dilute citric acid slowly added with magnetic stirring, ph measured and sample bottle sealed at 10:20 am 2

3 5) 1 L sample with 1 ml of the dilute citric acid added and immediately sealed and agitated at 10:30 am 6) 1 L sample with 5 ml of the dilute citric acid added and immediately sealed and agitated at 10:35 am 7) 1 L sample with 100 mg soda ash added, stirred and ph measured, followed by 5 ml of the dilute citric acid slowly added with magnetic stirring, ph measured and sample bottle sealed at 10:20 am FIGURE 1 Sample Water ph Reduction with Citric Acid Addition Each of the above 7 sample bottles were stored for 24 hours at room temperature in a dark closet. After 24 hours these samples were each used to fill 40 ml sample vials to be sent for lab testing for HAA and THM. The above sample test #1, #2 and #5 were also sent for lab testing of their ph, TOC and UV transmissivity. Results For all the sample titration tests; free chlorine analysis were measured using a Hack Dr2700 Colorimeter with Hack DPD Free Chlorine Reagent. ph was measured using a Thermo Scientific Orion Versa Star benchtop meter with a Thermo Scientific Orion ROSS Ultra ph/atc Triode Refillable Electrode. 3

4 The measurement values for the above tests were as follows: 1) Reference sample at sample time ph = 7.37, free chlorine = 1.29 mg/l after 24 hours ph = 7.01, free chlorine = 0.91 mg/l 2) 5 ml citric sol n at sample time ph = 7.11 after 24 hours ph = 6.65, free chlorine = 0.97 mg/l 3) 12 ml citric sol n at sample time ph = 6.73 after 24 hours ph = 6.32, free chlorine = 0.98 mg/l 4) 16 ml citric sol n at sample time ph = 6.52 after 24 hours ph = 6.10, free chlorine = 0.98 mg/l 5) 1 ml citric sol n after 24 hours ph = 6.98, free chlorine = 0.93 mg/l 6) 5 ml citric sol n after 24 hours ph = 6.72, free chlorine = 0.96 mg/l 7) 100 mg soda ash at mixing time ph = ml citric sol n at sample time ph = after 24 hours ph = 10.19, free chlorine = 0.76 mg/l The laboratory used for sample testing was CARO Analytical Service in Richmond, BC. The THM samples where preserved, however the samples provided for the HAA analysis were not preserved and were analysed 3 or 4 days after the sample was taken. The results of the laboratory analysis for HAAs and THMs are summarized in Table 1 below. Table 1 Sample Water Analysis Results Sample No Description Reference sample 1.8 mg/l citric added slowly, magnetic stirring 4.4 mg/l citric added slowly, magnetic stirring 5.9 mg/l citric added slowly, magnetic stirring 0.37 mg/l citric added, immediately capped 1.8 mg/l citric added, immediately capped 100 mg/l soda ash, 1.8 mg/l citric added slowly, magnetic stirring Initial Chlorine mg/l Initial ph Citric added (369 mg/l) ml Derived citric acid dose mg/l ph (citric dosed) ph (after 24 hrs) Chlorine (after 24 hrs) mg/l TTHM µg/l HAA5 (see note 1) µg/l UVt % TOC mg/l < <0.5 ph (lab) NOTE 1: HAA analysis were taken from unpreserved sample bottles approximately 3 or 4 days after sampling. 4

5 The citric acid concentration in the dilute citric acid solution is determined as follows: 0.6 ml citric acid x 50% concentration x 1.23 SG / 1000 ml water = 369 mg/l citric acid concentration in the dilute citric acid solution. The concentration of citric acid added to each of the sample tests are determined as follows: Tests #2, #6 and #7, citric acid conc. = 369 mg/l citric sol n x 5 ml/1000 ml water = 1.8 mg/l. Similarly for test #3 = 4.4 mg/l, test #4 = 5.9 mg/l and test #5 = 0.37 mg/l citric acid concentration. These derived values are also include in the Table 1 results. Interpretation For each of the above tests, 24 hours storage time was selected to allow sufficient time for the DBP formation reactions to reach substantial completion to be representative of water age in typical water supply systems. For each test, Table 1 reports the total THM s as the sum of the lab measurements for; Bromodichloromethane, Bromoform, Chloroform and Dibromochloromethane. As expected, Chloroform accounted for the vast majority (>96%) of the total THM s measured. Figure 2 below shows the composite results for the 7 sample tests performed. For ease of relative comparison the ph and free chlorine measurements initial values (at time of sampling) are shown as stacked columns over the final values (taken after 24 hours). FIGURE 2 Composite Test Results The results of the tests indicate that formation of THM (chloroform) and HAA s due to citric acid addition to chlorinated water are significant, but not alarming. The reference sample (without citric acid addition) had the lowest concentrations of DBP s, the presence of which is 5

6 assumed to have formed from NOM present in the treated water. The tests #2 and #6 had equal amount of citric acid addition (1.8 mg/l) which understandably resulted in similar residual chlorine concentrations and ph measurements after 24 hours water age. These citric acid test samples have a reduce chlorine decay which is expected for the lowered ph of these samples as compared to the reference sample. The DBPs levels also had similar measurements with test #6 slightly higher than test #2 which could possibly be due to some volatilization of DBPs from the test #2 sample during its magnetic stirring. Test #2 and #6 (1.8 mg/l citric acid) represent the maximum expected concentration of citric acid carryover in the Eagle Lake WTP clearwell based on the observed depression in clearwell ph after a citric acid CIP. For these test we find a 20% to 30% increase in THM and a 50% to 80% increase in HAAs from the reference sample. While this increase is noteworthy it is still well below the regulatory limits for this WTP. When the concentration of citric acid is significantly reduced as in test #5 (0.37 mg/l citric acid), the chlorine decay rate was much closer to the reference sample and the formation of THM was even higher (40% increase over reference sample) than the more concentrated citric tests #2 & #6. On the other hand, the increase in HAAs formation was not as pronounced (25% increase over reference sample) as was for tests #2 & #6. This result is important as it indicates that the levels of chloroform DBP may continue to rise for some time as the citric acid carryover is diluted through the distribution system. The tests #3 and #4 represent citric acid concentrations (4.4 mg/l and 5.9 mg/l) well in excess of the available chlorine (1.29 mg/l). These tests represent the higher concentration of citric acid which would be present from carryover immediately after a CIP cleaning. While such high concentrations would not be expected to persist for 24 hours, these tests were included to provide insight into whether excess citric could provide reason for concern. For these tests it is noted that the formation of THM (chloroform) was at approximately the same concentration as the reference sample. The reason for this in not known but is speculated to be due to the reduction in ph [Bond et al., (2012)] or possibly due to excess citric acid quenching of the chloroform formation reaction. The HAAs formation on the other hand showed increase in concentration in line with the other test samples performed (80% increase for #3 and 35% increase for #4 over the reference sample). Test #7 was carried out to provide insight into the effect of higher alkalinity water on citric acid carryover and the resulting DBP formation. For this test it was noted that the same concentration of citric acid as used in tests #2 & #6 resulted in a ph adjustment of only 0.01 (10.57 to 10.56) whereas test #2 resulted in a ph adjustment of 0.26 (7.37 to 7.11). This ph buffering capacity is as expected and could prevent citric acid carryover from being detected in membrane WTPs operating with higher alkalinity source water. For this test the free chlorine decay was considerably higher as expected due to the high ph [Summers et al., (1996)]. It was also noted that the THM (chloroform) formation was considerably higher (approximately 80% higher than the reference sample) while the HAAs where slightly lower (6% lower than reference) as expected [Hansen et al., (2012)]. Discussion The tests indicate that CIP solutions carryover can result in low concentrations of these solutions passing to the treated water and that in the case of citric acid these concentrations can result in an 6

7 increase in formation of DBPs. For this reason it would be advantageous to minimize the amount of carryover of CIP chemicals from membrane cleaning operations to the final treated water. Methods commonly employed to purge CIP solutions from membrane systems include back-flushing and/or product water dumping after membrane chemical cleaning. Back-flushing is when treated water flows into the membrane permeate headers providing reverse permeation through the membranes to displace the CIP solution from the membrane piping, headers and vessels. Product water dumping is when the initial water produced from a membrane system containing CIP solution is directed to waste until the CIP solution has been sufficiently flushed from the membranes and piping. Typically these flushing waters must be directed to neutralization where they can be properly treated and disposed. Carryover of CIP solutions can be reduced by increasing the volume of flushing water used to displace the CIP solution, however, this results in a trade-off between optimizing the volume of flushing water consumed vs. effectiveness in CIP solution displacement (reduction in carryover). Figure 3 below depicts the common arrangement of UF/MF system membrane unit header with graphic depiction of the imbalance of flushing flow between membranes along the header. FIGURE 3 Flushing Water Progression Through Membrane Unit As depicted in Figure 3, step 2), flushing water entering the membrane permeate piping will initially displace the CIP solution from the piping essentially as plug flow. However, as the flushing water reaches the lead membrane modules along the header, flushing water will branch flow to the lead module while CIP solution in the header continues to be displaced through the header towards the trailing membrane modules. As each membrane module is exposed to the advancing flushing flow, the flushing flow is further split and the remaining flushing flow through the header is successively reduced as it reaches more membrane modules. In this 7

8 arrangement, the lead membrane modules will receive more flushing water, for longer duration than the trailing membrane modules with the effect that the flushing water discharge will gradually decline from full CIP concentration at the start of flushing (plug flow through the piping) to decreasingly lower CIP concentration as flushing of membranes progresses along the header length. This is akin to a dilution process ( dilution flushing ) which can require an excessive amount of flushing water to achieve very low concentration of CIP solution carryover. A possible improvement to dilution flushing could be to first displace CIP solution from the entire header with plug flow by introducing the flushing water from one end of the header and expelling the CIP solution from the other end as depicted in Figure 4 below. Once the piping and header volume has been displaced the header discharge end could be closed and the flushing water directed through all the membrane modules at the same time. While this arrangement would reduce the imbalance of flushing water volume between each membrane module, it would require changes to the header and piping arrangement for existing plants and would not improve any flow imbalance which may exist inside each membrane module or dead-legs in the piping. FIGURE 4 Displacement Flushing by Plug Flow Another alternative to the current dilution flushing practice could be to implement air-purge displacement flushing whereby CIP solutions would be displaced from the membranes and membrane header by air as depicted in Figure 5 below. This could be accomplished either by draining the system while admitting air (vacuum breaking) to the permeate side, or by driving air into the system using the MIT air supply system to permeate the CIP solution through the membrane. Such displacement flushing could be relatively easily applied to many existing UF/MF plants using their existing equipment and configuration just by altering their control strategy. After purging CIP solution from the membrane unit with air, a modest volume of flushing water could be applied to complete effective flushing of the membrane unit and thereby significantly reduce carryover of CIP solution (such as citric acid) to the final treated water. The flushing water could be introduced with vacuum priming or forward-flush permeation or other method as best suited to the particular plant arrangement. 8

9 FIGURE 5 Air-Purge Displacement Flushing Conclusion In UF/MF membrane WTPs, CIP solution carryover to product water is unavoidable and in the case of citric acid can affect the treated water quality by reaction with disinfection chlorine to create DBPs. In order to minimize DBPs in the treated water it would be advantageous to minimize such carryover of CIP solutions. A proposal has been offered to improve flushing and purging of CIP solutions from membrane WTP in order to minimize the quantities of CIP solution carryover. The authors recommend a further study to perform testing and measurements of the proposed air-purge improved flushing on a full scale membrane WTP in order to assess the effect on reduction of carryover, final water quality and ease of implementation of the proposed improved flushing method. References Tom Bond, Emma H. Goslan, Simon A. Parsons and Bruce Jefferson (2012), A critical review of trihalomethane and haloacetic acid formation from natural organic matter surrogates, Environmental Technology Reviews Vol. 1, No. 1, November, Kamilla M. S. Hansen, Sarah Willach, Maria G. Antoniou, Hans Mosbæk, Hans-Jørgen Albrechtsen and Henrik R. Andersen (2012), Effect of ph on the formation of disinfection byproducts in swimming pool water Is less THM better?, Water Research, Vol. 46, Issue 19, December, R.Scott Summers, Stuart M. Hooper, Hiba M. Shukairy, Gabriele Solarik, and Douglas Owen (1996), Assessing DBP yield: uniform formation conditions, Journal of American Water Works Association Vol. 88, No. 6, June,